1. Field of the Invention
This invention is directed to a high voltage feedthrough particularly useful for ion pumps.
2. Description of Related Art
One class of vacuum pump is the ion pump. Both active and inert gases are pumped by precipitation or adsorption following molecular dissociation, gettering by freshly sputtered cathode material, surface burial under sputtered cathode material, ion burial following ionization in the discharge, and/or fast neutral atom burial. The ion pump is basically a low pressure cold cathode Penning discharge. In a Penning cell, the electric fields trap electrons in a potential well between two cathodes, and the axial magnetic field forces the electrons into circular orbits to prevent their reaching the anode. This combination of electric and magnetic fields causes the electrons to travel long distances in oscillating spiral paths before colliding with the anode. These long paths result in a high probability of ionizing collisions with gas molecules. An excellent description of the operation of ion pumps, with dimensions and operating parameters similar to those encountered in a 0.2 liter per second ion pump has been given by Wolfgang Knauer in "Mechanism of the Penning Discharge at Low Pressures," Journal of Applied Physics, Volume 33, Number 6, pages 2093 through 2099 (June, 1962 ).
As a result of this activity at the cathode, sputtering occurs. The sputtered material, having a neutral charge, travels in a straight line from the point of sputtering.
The high voltage feedthrough feeding the anode includes a ceramic insulator which is exposed to the interior of the pump. In the conventional pump, sputtered material deposits on the ceramic insulator. After a sufficient time has elapsed, a conducting layer of cathode metal builds up. This layer short-circuits the anode to the main body of the pump which is at cathode potential. Because sputtering is directly proportional to the anode current, the life of the pump is directly proportional to the total charge which has flowed through the anode circuit.
One solution to the problem has been to place a flat disc on the anode lead, inside the cathode, to shield the ceramic from sputtering. However, in view of the tubular nature of the vacuum housing, adequate shielding has not been possible because the curved surface of the vacuum housing comes too close to the flat shield disc when the disc is of adequate size to provide proper shielding. Therefore, there is need for a construction wherein sputtering is prevented from creating a short circuit path along the anode feedthrough insulator.